First Android app in Android Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What will this Android app display?

Consider this simple Android app code snippet. What text will appear on the screen when the app runs?

Android Kotlin

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.Text
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.runtime.Composable

class MainActivity : ComponentActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContent {
      Greeting("Android Learner")
    }
  }
}

@Composable
fun Greeting(name: String) {
  Text(text = "Hello, $name!")
}

A Hello, World!

B Hello, Android Learner!

C Greeting Android Learner

D App crashed with error

💡 Hint

Look at the Greeting composable and what string it uses.

The Greeting composable displays the text "Hello, $name!" where name is "Android Learner" passed from setContent. So the screen shows "Hello, Android Learner!".

❓ lifecycle

intermediate

When is onCreate() called in an Android app?

In the Android app lifecycle, when does the onCreate() method run?

A When the app is closed by the system

B Every time the app goes to background

C When the app is first launched and the activity is created

D When the user presses the back button

💡 Hint

Think about the first step when an activity starts.

onCreate() is called once when the activity is first created, before it becomes visible to the user.

📝 Syntax

advanced

What error does this Kotlin code produce?

Look at this Kotlin code snippet for an Android app. What error will it cause?

Android Kotlin

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.material3.Text

class MainActivity : ComponentActivity() {
  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContent {
      Text(text = "Welcome")
    }
  }

  fun Text(text: String) {
    println(text)
  }
}

A Compilation error: Conflicting function names for Text

B Runtime error: NullPointerException

C No error, app displays "Welcome" text

D Syntax error: Missing import for Text

💡 Hint

Check if defining a function named Text conflicts with Compose's Text composable.

Defining a function named Text in the same scope conflicts with the Compose Text composable, causing a compilation error due to ambiguity.

❓ navigation

advanced

What happens when this navigation code runs?

Given this Android Jetpack Compose navigation snippet, what screen will the user see after clicking the button?

Android Kotlin

import androidx.compose.material3.Button
import androidx.compose.material3.Text
import androidx.navigation.NavController

@Composable
fun FirstScreen(navController: NavController) {
  Button(onClick = { navController.navigate("second") }) {
    Text("Go to Second")
  }
}

@Composable
fun SecondScreen() {
  Text("Second Screen")
}

A The app navigates to SecondScreen and shows "Second Screen" text

B The app stays on FirstScreen and shows the button

C The app crashes with navigation error

D The app shows a blank screen

💡 Hint

Look at what navController.navigate("second") does.

Calling navController.navigate("second") moves the app to the SecondScreen composable, which displays "Second Screen" text.

🧠 Conceptual

expert

Why use setContent in an Android Compose app?

What is the main purpose of calling setContent in an Android app using Jetpack Compose?

A To start a background service for the app

B To set the app's theme colors and styles globally

C To register event listeners for user input

D To define the UI content of the activity using composable functions

💡 Hint

Think about how Compose replaces XML layouts.

setContent replaces the traditional XML layout by setting the UI content directly with composable functions in Kotlin code.